ET = ET is Einpresstiefe which is German for Offset. This is how far the wheel’s mounting point is from the middle.

Hub = The diameter in mm of the center hole in the wheel.

Difference = Deviation in percentage of the circumference compared to the original circumference. This percentage is also the difference in speed on the speedometer.

How to use the tolerances Re-check application?
The calculated sizes are limited per car to a maximum in 1. Width and 2. Circumference deviation.

A wheel with a very wide tire for example 335/20/15 might have a similar circumference as a mid-sized hatchback but will never fit. Therefore the maximum width is limited both ways (smaller&wider) compared to the stock tire-size. If you believe the tolerance is too tight or to loose you can adjust it to what you think is right. Or just for the fun of it.

Every nation has their own laws on how much an after-market wheel and tire may vary from the manufacture original. The database has therefore limited the difference to a percentage (smaller & bigger) that is commonly known as acceptable. If you country has strict rules, or the opposite you want to go extreme, you can change the limitation of the % that the calculator uses. The difference inÂ % Â is the difference in circumference meaning that this difference is similar to the difference on the speedometer. Just play with it, and you will see the difference in results.

Width relation. What is the relation between the tire and wheel width?The width of the tire and rim are not similar. When you compare the tire millimeters to the to the width of the rim in inches there is a difference. This is because a wide tire can sit on a smaller wheel like a balloon (mostly series 60/70/80 tires on 4×4’s) or a smaller tire can be fitted stretched on a wide wheel (like the low profile tires like you see on racecars series 40/30/25). There is no calculation for this so you will have to check with a specialist to see what combinations are possible. The Wheelsizecalculator.com results give you an estimate what is possible per tire size.

Circumference.What is the circumference and how do you calculate it?
The circumference is the length of the outline of a tire. If you would cut the tire and lay it flat on the ground and measure the length then that would be the circumference. Because you cannot cut every tire, we use a formula. The formula is diameter x Pi=circumference. Pi is a Greek math symbol and stands for 3.14 followed by a ridicules amount of other numbers that your calculator will remember for you. The diameter that is on the tire wall is the inner diameter; you will need the total diameter for this formula. You can imagine that if the circumference of a tire is bigger (the length is longer) it will take a longer time to make a complete roll. To fit different size tires (+matching wheels) on your car you will need a tire with the same circumference. A difference in circumference of -2% up to 4% is often tolerated.

Questions about wheels

J-size. Why is the width of a wheel given as J?
On a Wheel (also called rim or alloy) you can find information such as J8x18 ET35. The J8 stands for the shape and width of the rim. The J shape is the most common shape and therefore the width is mostly given as a J-size. The 8 is 8 inches.

Einpresstiefe. What does ET mean?
ET is einpresstiefe meaning offset and gives the amount of mm that a wheel sits from it’s mounting point. When the offset is 0 the mounting point is exactly in the middle of the wheel. The lower the offset size the further the wheel sticks out. For example, if your car stock offset is 40 and you installed wheels with an offset of 10, the wheels would stick out 30 mm more than stock. The basic tolerance for different offset sizes is 5 mm.

PCD. What is the PCD?
PCD is the pitch circle diameter and stands for the amount of bolts in a circle with a diameter in mm. For instance, a PCD of 5/112 means that there are 5 bolts in a circle with a diameter of 112 mm to attach the wheel to the car.

HUB. What is the hub size?
In the calculator database you will find that there are PCD and hub sizes available for many cars. The hub size is the center hole of the wheel that fits over the car’s hub. A plastic plate called the hubcap covers the hub.

Circumference. What is the circumference and how do you calculate it?
The circumference is the length of the outline of a tire. If you would cut the tire and lay it flat on the ground and measure the length then that would be the circumference. Because you cannot cut every tire, we use a formula. The formula is diameter x Pi=circumference. Pi is a Greek math symbol and stands for 3.14 followed by a ridicules amount of other numbers that your calculator will remember for you. The diameter that is on the tire wall is the inner diameter; you will need the total diameter for this formula. You can imagine that if the circumference of a tire is bigger (the length is longer) it will take a longer time to make a complete roll. To fit different size tires (+matching wheels) on your car you will need a tire with the same circumference. A difference in circumference of -2% up to 4% is often tolerated.

Mounting. How to mount a wheel on a car?
A wheel can be mounted by first tightening the bolts with your hands and then using a power tool. It’s important to follow the sequence as displayed in the picture. It is advisable to use a torque wrench set to 100-150 Nm of torque depending on your car’s specification

Balancing. When do you balance a wheel?
Manufacturing a tire or wheel with a fully balanced distribution of material is technically almost impossible. Besides this, dirt and wear spots imbalance the wheel and tire as well. It is therefore necessary to balance each wheel/tire before mounting to the car and to check the balance regularly. Balancing is done on a balancing machine. The machine tells the operator where extra weight must be placed on the wheel to get it balanced. A modern method is to put magnetic lead on the inside of the wheel out of sight. The old-fashioned method is to clamp weighted clips on the outside edge of the wheels rim. The last method is certainly not advisable on nice set of wheels. The rim is a part of the wheel, but the word rim is often used for the complete wheel.

Wheel Bolts. Are all wheel bolts the same?
Most wheel bolts are galvanized steel but there are also chrome plated steel bolts and titanium ones. The head of the bolt usually has a smooth conical piece with a solid ring. The length of the bolt varies by car. In the database on the homepage you will find the stock bolt sizes for many cars. M14 or M12 are the most common thread sizes. M stands for metric and the number for the thickness in mm. On the bolt head you can find an ISO code indicating the grade of steel (tensile stress and yield stress). Wheel Bolts are usually conical which means that the bolt slowly becomes thicker and will fasten itself. The bolts can also be flat with the proportional thread thickness. If you use spacers, you logically need longer bolts.

Adjustments. How to adjust the car fender to fit bigger wheels when the car is lowered?
You need to bend the fender outward by using a fender roller. A fender roller is a tool that fits on the cars wheel mounting spot and has a shaft and a wheel. The wheel is used to roll the fender outwards. You need experience and patience otherwise you will damage the fender and paint. Basically it goes as follows; jack up the car, take off the wheel, mount the fender roller, warm up the paint with a blower and slowly roll out the fender.

Aluminum. Are alloy wheels better?
Alloy wheels are made from aluminum or magnesium metals (or sometimes a mixture of both). Alloy wheels differ from normal steel wheels because of their lighter weight. This improves the performance of the car. However, some alloy wheels are heavier than the equivalent size steel wheels due to poor build quality. Quality alloy wheels are better heat conductors than steel wheels. This improves the heat dissipation from the brakes, which reduces the chance of brake failure in more demanding driving conditions. While alloy wheels are available standard on higher-priced model vehicles, they are not standard on many cars. Alloy wheels give you the opportunity to individualize your car. Mind that, alloy wheels are prone to galvanic corrosion if appropriate preventive measures are not taken.

Questions about the suspension.

Alignment. What is aligning a car?
Aligning a car means putting all the wheels in the correct position so the car will go straight without touching the steering wheel. The funny fact is that wheels are never positioned straight in order to go straight. Most wheels sit at an angle compared to the car. This is necessary because there are various forces on the wheels due to the drive train, steering and mass of the car. If we look at the vertical position of a wheel from the front of the car we are talking about the camber. If you look at the wheels from the front and the wheels point inside on where they contact the ground then that is positive camber. If the wheel points outward it is negative camber. Camber determines whether a car tends to under steer or over steer. Seen from above, the wheels can also point inwards or outwards into the driving direction. Driven wheels tend to want to go outside due to the force of the engine. To overcome this effect the driven wheels are pointed inwards. In a rear wheel drive car the front of the rear wheels point towards the car. This is called toe in. In a front-wheel drive car the rear of the front wheels point outwards from the car. This is called toe out. It’s important that the alignment of the car is correct; otherwise, it will not go straight by itself, tear the tires and make for an uncomfortable drive.

Springs. Why change the springs?
If you are changing the original rims and tires with bigger rims with lower series tires you might want to consider changing the shocks and springs as well. Because the hysteresis in the tire itself decreases with lower series tires, the springiness of the wheel will increase. To dampen this up and down bouncing motion, you will need stiffer springs. New springs will allow you to lower your car as well for lowering the point of gravity or show function.

Shock absorbers. Why change the shocks?
Springs alone are not shock absorbers as springs only store and do not dissipate or absorb energy. Vehicles typically employ both springs (or torsion bars) and hydraulic shock absorbers. Because the springs and shocks work together, it is advisable to purchase a complete set of shocks and springs. This advice applies especially for the MacPherson strut suspension system where the spring and shock absorber are combined in the one suspension unit.

Bars. What is the purpose of strut and anti roll bars?
A strut bar is car suspension accessory used in conjunction with MacPherson struts on monocoque or unibody chassis to provide extra strength between the strut towers. The strut bar is designed to resist longitudinal compression, meaning limiting the torsion / flex of the chassis by keeping the two wheels’ suspension components separate. This improves the handling by reducing under steer, tire wear and metal fatigue in the strut tower area. The strut bar is also a nice accessory for show under the hood. A sway bar (also known as stabilizer bar or anti- roll bar) is a torsional spring that resists body roll motions. It is constructed out of a U shaped piece of steel tube that connects to the body at the left and right side of the suspension. If the left and right wheels move together, the bar just rotates about it’s mounting points and does not bend. If the wheels move relative to each other, the bar is forced to twist. A sway bar increases the resistance to roll in turns through its stiffness. The stiffness of an sway bar is based on diameter, material, length, geometry and rigidity of the bar’s mounting points. Some anti-roll bars are adjustable, allowing their stiffness to be altered by increasing or reducing the length of the lever arms without replacing the entire bar. The stiffer the bar, the more force required to move the left and right wheels relative to each other. This increases the amount of force required to make the body roll.

Spacers. When do you use spacers?
The spacer is mounted behind the rim to increase the space between the rim mounting point and the rim itself. This can be applied for correcting the offset of an aftermarket rim or to improve the ride. Make sure that the wheel does not stick out of the chassis too much because in many countries this is not allowed and the tires will destroy the paint by spraying dirt and gravel over it.